1. Field of the Invention
The present invention relates, in general, to spacer grids used for placing and supporting fuel rods in nuclear reactor fuel assemblies and, more particularly, to a spacer grid with both H-springs, individually having a curved middle portion conformal with the surface of each fuel rod, and a positioning dimple having the same radius of curvature as that of each fuel rod, thus being brought into conformal surface contact with the fuel rods and thereby accomplishing soundness of fuel rods within the fuel assembly regardless of any impact acting on the fuel rods, the grid also effectively protecting the fuel rods from fretting corrosion even when gaps are formed between the fuel rods and the positioning springs and dimples.
2. Description of the Prior Art
As well known to those skilled in the art, spacer grids are positioning elements for placing and supporting a plurality of elongated fuel rods within a nuclear fuel assembly. In each of the spacer grids, two sets of intersectional grid strips are arranged in sets at an angle to each other prior to being encircled by perimeter strips, thus forming a plurality of four-walled cells for fuel rods. In each of the cells of a spacer grid, a plurality of positioning springs and dimples are provided on the strips, thus elastically placing and supporting each fuel rod within the cell. When the support force provided by the positioning springs and dimples is less than a specific level, the springs and dimples fail to effectively place and support the fuel rods at desired positions, thus failing to accomplish soundness of the fuel rods within the fuel assemblies. On the contrary, when the support force provided by the positioning springs and dimples is exceedingly higher than the reference level, the springs and dimples undesirably cause frictional damage, such as scratches, to the surfaces of the fuel rods when inserting the fuel rods into the cells of the spacer grids. The positioning springs and dimples, with such exceedingly high support force, also fail to allow the fuel rods to smoothly slide on the springs and dimples when the fuel rods are elongated due to neutron irradiation induced growth during an operation of a reactor. In such a case, the fuel rods are bent.
When the fuel rods are undesirably bent as described above, the fuel rods reduce the intervals between them or come into contact with each other, thus reducing the cross-sectional area of the coolant passages defined between the fuel rods within the fuel assembly or blocking said passages. This prevents heat from being effectively transferred from the fuel rods to coolant, thus causing the fuel rods to be partially overheated and thereby causing a nuclear boiling separation of the fuel rods. This results in reduction in the output power of the nuclear fuel.
Nuclear fuel, having a high burn-up and zero defect, has been actively studied recently. The primary method of producing the nuclear fuel having a high burn-up is to improve the thermal property of the nuclear fuel. That is, the fuel assembly is designed to improve the heat transferring effect between the fuel rods and the coolant. In order to accomplish the above object, the flow property of the coolant about the fuel rods has to be improved. Such an improvement in the flow property of the coolant may be accomplished by changing the structural design of the spacer grids. For example, the spacer grids may be provided with so-called "mixing deflectors" or "vanes". The flow property of coolant may be further improved in accordance with the configuration of such mixing deflectors or vanes. In addition, the flow property of the coolant is also improved by designing the coolant passages to allow the coolant to more effectively flow within the assembly.
However, the means for improving the thermal property of the nuclear fuel is designed to form a turbulent flow of the coolant about the fuel rods and this causes the elongated fuel rods to be vibrated during an operation of a reactor. Such a vibration of the fuel rods causes the fuel rods to repeatedly slide on the positioning springs and dimples of the spacer grids, and so the fuel rods are partially abraded at the contact surfaces where the fuel rods are brought into contact with the positioning springs and dimples of the grids. That is, the vibration of the fuel rods, caused by the turbulent flow of the coolant, results in fretting wear of the fuel rods. It is thus noted that the means for improving the thermal property of the nuclear fuel also undesirably causes damage or fretting wear to the fuel rods. The above-mentioned fretting wear is caused by a fatigue fracture or a fatigue wear which is caused by cracks and growth of said cracks, the cracks being formed on the contact surfaces of the fuel rods when the fuel rods are vertically and/or laterally vibrated with infinitesimal amplitude relative to the positioning springs and dimples while being pressurized by a contact pressure at the contact surfaces between the fuel rods and the positioning springs and dimples.
In a nuclear fuel assembly, the fuel rods are placed and supported within cells of grids by positioning springs and dimples as described above. In addition, the contact pressure, acting on the contact surfaces of the fuel rods, are caused by both the spring force of the positioning springs and the reaction force of the positioning dimples. The infinitesimal amplitude vertical and/or lateral vibration of the fuel rods is caused by the turbulent flow of the coolant within the fuel assembly. From a microscopic point of view, such a fretting wear is the growth behavior of cracks, and so the fuel rods can be free from such fretting wear when the spacer grid is designed to protect the contact surfaces of the fuel rods from such cracks.
FIG. 1 is a perspective view, showing the construction of a typical nuclear fuel assembly. As shown in the drawing, the fuel assembly 1 comprises a plurality of elongated, closely spaced and parallel fuel rods 9. The fuel rods 9 are placed and supported within the assembly 1 by a plurality of spacer grids 7 and 8. The top and bottom of the fuel assembly 1 are covered with pallets 2 and 3, respectively, and so the assembly 1 is protected from any external loads acting on the top and bottom thereof. The pallets 2 and 3 and the spacer grids 7 and 8 are integrated into a single structure using a plurality of guide tubes 4. A framework of the assembly 1 is thus fabricated. In the above assembly 1, the spacer grids 7 and 8 are laterally arranged along the axes of the guide tubes 4 while being regularly spaced from each other. The above spacer grids 7 and 8 are welded to the guide tubes 4, thus being integrated into a fuel assembly 1.
FIGS. 2a and 2b show a typical Inconel spacer grid for use in nuclear fuel assemblies. The above spacer grid 7 is fabricated by arranging two sets of intersectioal grid strips at right angles to each other, thus forming a plurality of four-walled cells. The above cells include two types of cells: fuel rod cells 13 used for placing and supporting the fuel rods 9 within the grid 7 and guide tube cells 14 used for placing and supporting the guide tubes 4 within said grid 7. Each of the fuel rod cells 13 has two positioning springs 18 and four positioning dimples 19, thus forming six support points. Each fuel rod 9 is thus supported by an associated fuel rod cell 13 while being brought into point contact or linear contact with the cell 13 at six points.
FIG. 3 is a perspective view, showing a fuel rod 9 which is placed and supported by a spacer grid while being brought into point or linear contact with an associated fuel rod cell at six points. As shown in the drawing, at the initial stage, the positioning springs 18 and dimples 19 have a stable support structure without having any gap at the contact points between the fuel rods and the springs and dimples. However, a gap may be formed at each of said contact points during an operation of the fuel assembly, thus breaking the stable support structure of the positioning springs and dimples. Such a gap causes the spacer grid to lose its original function of placing and supporting the fuel rods within the fuel assembly.
FIGS. 4a and 4b show the configuration of the typical positioning springs and dimples of a spacer grid used for supporting the fuel rods. As shown in the drawings, the positioning springs 18 and dimples 19 are brought into nonconformal contact with the fuel rods 9. Such a non-conformal contact between the fuel rods 9 and the positioning springs 18 and dimples 19 may cause damage to the fuel rods 9 due to a contact load repeatedly applied to the fuel rods 9 when a gap is formed at the contact points between the fuel rods 9 and the positioning springs 18 and dimples 19.
It is thus preferable to design the spacer grids to effectively protect the fuel rods from fretting wear and to stably support the fuel rods within the cells without failure for the expected life span of the fuel rods.
In order to protect the fuel rods from fretting wear, the spacer grid has to be designed to be free from causing such a fretting wear of the fuel rods. However, it has been noted that the fretting wear is primarily caused by a gap undesirably formed between the fuel rods and the positioning springs and dimples. Such a gap is formed due to (1) a reduction in the spring force of the positioning springs due to neutron irradiation during an operation of the reactor, (2) a difference in the thermal expansion between the fuel rods and the spacer grids, and (3) a reduction in the diameter of the fuel rods due to cladding creep of each fuel rod. When such gaps are formed between the fuel rods and the positioning springs and dimples, the fuel rods are repeatedly moved relative to the springs and dimples, thus coming into intermittent contact with the springs and dimples. This causes fretting wear of the fuel rods.
In order to allow the spacer grids to stably support the fuel rods, thus accomplishing soundness of the fuel rods within a fuel assembly for the expected life span of the fuel rods, it is necessary to design the positioning springs to elastically support the fuel rods against the positioning dimples with a sufficient spring force. However, it has been noted that the positioning springs and dimples gradually lose the designed spring force due to neutron irradiation during an operation of a reactor. This forms gaps between the fuel rods and the positioning springs and dimples. The gaps cause the fuel rods to be vibrated vertically and/or laterally due to a circulation of coolant within a fuel assembly. It is, thus, impossible to accomplish soundness of the fuel rods within the fuel assembly.